Electrical connectors are designed to provide conductive paths between adjacent printed circuit boards. Some connectors also mechanically seize the boards to which they are connected so as to physically secure one board to the adjacent board. Connectors of this type are often installed on a primary, or "mother" board, and are adapted to receive the edges of secondary, or "daughter" boards. These connectors are called edge connectors and are used in modern electrical eguipment that contains a number of parallel daughter boards that are closely packed together.
Edge connectors often comprise a number of conductive contacts that are spaced apart and arranged linearly in a housing. Each contact is metallic, and is positioned to abut a conductive contact pad on the edge of the daughter board. Often the contacts are arranged in two parallel rows so the daughter board can be inserted therebetween. When a daughter board is positioned between the rows, the contacts exert a gripping force on the daughter board so as to secure it in the housing.
There are a number of disadvantages to the edge connectors currently in use. The conductors of most of these connectors have vertical stems that must be solder-connected to plated through holes in the mother board. Providing the mother board with a large number of plated through holes consumes a significant area on the board and reguires that conductors and other circuit components on the board be designed around them. Moreover, it is difficult to change the edge connectors on a board since they are semi-permanently attached to the board.
In addition, the mechanism many edge connectors use to secure the daughter boards is inefficient. Some edge connectors rely on Zero Insertion Force, (ZIF) mechanisms. The contacts of these connectors are in registration with a cam rod so that at least one of the parallel rows of contact connectors can be selectively moved towards or away from the opposite row. Initially, the contacts are spaced apart from each other. After the daughter board is inserted between the opposed rows of contacts, the rows are moved together so as to grip the daughter board therebetween. ZIF connectors rely on relatively expensive mechanica1 mechanisms to secure the daughter boards. Furthermore, the securing mechanism is formed of a number of moveable parts, any of which may malfunction because of either wear or breakage.
Other edge connectors rely on Low Insertion Force, (LIF) contacts. These contacts are pieces that have been stamped and bent to have a shape with spring-like resiliant characteristics. Eventually though, the contacts lose their resiliency and are deformed into a permanently open shape. When a daughter board is placed between the worn contacts they do not firmly abut the daughter board. As a result, they no longer secure the daughter board to the housing, nor do they make a reliable electrical connection with the daughter board's contact pads.
Moreover, only a limited number of electrical connections can be made per unit length of the daughter board. This is because the individual contact pads on the daughter board have to have a minimum width to insure that there is a sufficient area of contact between them and the conector contacts to form a continuous electrical path with minimal resistance. Also, the contact pads must be spaced apart a sufficient distance so that under normal operation conditions adjacent pads will not short circuit. Current contact pads have a cross-sectional width of 80 mils (0.080") and are spaced apart approximately 20 mils. Thus, each contact pad and insulating gap occupies 100 mils of length, so a maximum of 10 contacts per inch of daughter board can be accommodated. The increasing miniaturization of electronic circuits requires that more connections per unit length of board be made available.
Furthermore, some edge connectors provide only signal contacts to the daughter board. They are not suited to transfer the power needed to operate components that may be located on the daughter boards.